Although there is a great deal of information on the rejoining of X or gamma-ray induced single stranded DNA breaks, no information exists on the production and rejoining of single and double stranded DNA breaks in cultured mammalian cells using high L. E. T. irradiation. This study will provide information for the estimation of R. B. E. (relative biological effectiveness) and O. E. R. (oxygen enhancement ratio) based on the production of strand breaks for cultured mammalian cells. The kinetics of rejoining of DNA breaks will be measured for fast neutron irradiated cells (with appropriate X-ray controls). Experiments with cells in medium, at reduced temperatures, and in the presence of DNP will be conducted. Single cell survival studies will be performed (1) to establish O.E.R. and R.B.E. for survival and (2) to measure the kinetics of repair of sublethal injury, for our particular beam. We plan to determine the O.E.R. and R.B.E. values for the production of breaks in synchronized cells. Both the mitotic selection and double thymidine block methods will be used to synchronize the cells. The studies to be performed will provide an additional insight into the effects of high L.E.T. radiation at the molecular level, which is of importance in both radiotherapy and radiobiology.